You could ask an IP lawyer for £12 on
www.justanswer.com
You can also look on
[url=http://law.freeadvice.com/intellectual_property/copyright_law/
]law.freeadvice.com/intellectual_property/copyright_law/
[/url]
to see how copyright works.
In principle the people who used your work could reasonably know that the work you delivered to them was not their copyright, unless it was in the contract that they owned it. If like in this case it was not and if they never paid you for the work, they should not be able to use it. In future, you should put a copyright disclaimer on all your work and stipulate in contracts that the copyright and other intellectual property rights are yours and only with your permission can the client use the work. As the websites show you, there are places you can register your work so it becomes easier to prove if you do go to arbitration or court.
In my opinion there are two things you can do right now, you can tell your client they have violated your copyright and you want payment of a certain sum. If they don't oblige, you could sue. However the latter option is so expensive, it is probably not worth while and the client will know that. Probably a case of once bitten, twice shy, better luck next time...
I hope this helps.